We will study the physiological role of prolactin (PRL) in the regulation of the function of the mammalian testis. The experiments will be conducted using two animal models: golden hamsters exposed to different photoperiods, and hereditary dwarf mice. In the hamster, exposure to a short photoperiod causes testicular atrophy and a reduction in plasma PRL levels. Administration of PRL to gonadally regressed hamsters stimulates specific LH binding by the testis, testosterone production and testicular growth resulting in restoration of fertility. Dwarf mice are congenitally PRL-deficient and sterile and can be made fertile by treatment with PRL. To determine the mechanisms by which PRL can stimulate the function of the testis, we will study the effects of PRL on testicular LH and PRL receptors, on the sensitivity of the hypothalamo-pituitary system to feedback regulation by gonadal steroids, on the release of FSH from the pituitary, on th responsiveness of peripheral target tissues to androgens and on the metabolism of testosterone. The interaction of PRL with other anterior pituitary hormones will be evaluated in intact hamsters during testicular regression and recrudescence and in hypophysectomized animals. The requirement for PRL in the stimulation of testicular growth by long photoperiods will be studied by pharmacological blockade of endogenous PRL release in animals returned from a short to a long photoperiod. A more complete understanding of PRL action on the pituitary-gonadal axis is important, because results obtained in other laboratories indicate that PRL may be normally involved in the normal regulation of testicular functions in several species including the human.